Thryotropin-releasing hormone (TRH) is a tripeptide produced in the hypothalamus, other central nervous sites, and the gastrointestinal tract, especially the pancreas. TRH stimulates TSH synthesis and release. Most circulating TRH, which is reapidly degraded, is probably derived from extra-hypothalmic tissues. Circulating TRH levels are increased in fetal and neonatal serum because of increased TRH production from fetal gut tissues and placenta and low levels of TRH-degrading activity in fetal blood. Plasma TRH levels have not been shown to be useful in diagnosis of any known thryroid condition.